This invention relates to circuits for producing, in response to an input signal, an output signal of a predetermined percent time duration of the period of the input signal. More particularly, the invention relates to a circuit suitable to be utilized as an integrator circuit for use in solid state ignition systems.
It has been recognized that present day mechanical ignition systems for automobiles or the like cannot meet future requirements for low level pollutent emission from combustion engines especially for long durations of operation. It has also been recognized that solid state ignition systems can be utilized to electronically control ignition firing to reduce both wasting of fuel and pollution caused by internal combustion engines for long periods of operation.
Many prior art solid state ignition systems have been proposed for internal combustion engine vehicles. Some prior art ignition systems employ at least one integrating circuit for producing, in response to timing signals generated in timed relationship to the engine, a control signal for charging and discharging the ignition coil at the required time to maintain firing in the engine. At least one prior art system charges and discharges an integrating capacitor at predetermined charging and discharging rates to produce an output signal which is of a predetermined percent time duration of the period of the input signal. Moreover, the initiation of the output signal occurs as the magnitude of successive timing signals cross a zero axis in a positive sense. One important function of the output signal is to positively inhibit charging of the ignition coil during the time duration of ignition coil discharge so to prevent false firing in the engine due to an undesirous input noise signal which can occur during the engine firing cycle.
The above prior art solid state ignition circuit employs an integrating capacitor connected between two reference points to provide the output signal discussed above. Because the ignition system is typically placed under the hood of automobiles or the like, the circuit employing a capacitor connected between two reference points is susceptible to noise transients produced during the normal operation of the engine, since these noise transients could be either inductively or capacitively coupled to the capacitor, a false signal could occur to charge the ignition coil during the firing cycle. If a false signal should be generated, false firing could occur during the firing cycle which could damage or at least seriously degrade the engine's performance.
Therefore, a need exists to provide a solid state ignition system comprising an integrator or timing circuit which utilizes a capacitor but is not susceptible to inductively or capacitively coupled noise transients produced by an internal combustion engine.